The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Ultraviolet (UV) and vacuum ultraviolet (VUV) radiation is generally defined to encompass the 200-400 nm and 100-200 nm wavelength regions, respectively, of the electromagnetic spectrum. Because the energies of VUV photons, for example, can be as large as approximately 12.5 eV, UV/VUV radiation is capable of initiating photochemical reactions that are inaccessible to optical sources emitting radiation of longer wavelengths and, therefore, lower photon energies. Since the strengths of many of the most important chemical bonds (e.g., C—H, O—H, etc.) are less than 10 eV, the commercial application of photochemical reactions hinges on the development of efficient and powerful sources of UV and VUV radiation.
Photochemical reactions that occur in the UV spectral region are responsible for many processes that have considerable medical and industrial value. Examples of such processes include the synthesis of Vitamin D and three-dimensional (3D) printing or stereolithography. Deep UV/VUV radiation is also effectively used to deactivate biological pathogens, disinfect water, clothing, and other surfaces, and desorb contaminants and hydrocarbons from otherwise clean surfaces, such as equipment devoted to semiconductor device fabrication. In addition, the use of UV radiation to disinfect a wound or surgical incision is believed to accelerate the healing process and hinder the occurrence of hospital-acquired infections. Most applications that use UV/VUV radiation owe their existence to the development of incoherent optical sources that emit radiation at wavelengths lying between about 185 nm and 350 nm. Although lasers are presently available at several wavelengths that fall within this spectral region (e.g., F2=157 nm; ArF=193 nm, KrCl=222 nm, KrF=248 nm, and XeCl=308 nm), these lasers offer little benefit in most industrial and medical applications due to their optical coherence, physically large size, cost (capital and operating), and inefficiency. For example, an argon fluoride (ArF) laser capable of producing 10 W of average power at 193 nm (100 mJ/pulse, operating at a pulse repetition frequency (PRF) of 100 Hz) is a formidable system. This type of laser is also quite large, expensive, heavy and, at a PRF of 100 Hz, requires maintenance after every few hundred hours of operation. In addition, the mean time between failure (MTBF) for commercial systems incorporating conventional lasers is generally limited by the laser itself. Therefore, although UV/VUV lasers have proven to be pivotal to several medical applications (such as the corneal refractive correction procedure known as LASIK, and the treatment of psoriasis), for example, lamps are the preferred solution for industrial applications if the requisite power and efficiency are available at the desired wavelength.
Despite the commercial potential of UV/VUV photochemistry, disinfection, and decontamination, the applications of 100-400 nm radiation have thus far been constrained by the generally low output powers available from conventional lamps. Because the optical power delivered by any UV/VUV lamp translates directly into the rate at which a photochemical or disinfection process proceeds, it is essential that lamps scalable to at least 1-10 W of average power be available in order for industrial and biomedical photochemical processes to reach their full potential. Indeed, the realization of high power, efficient lamps in the 100-400 nm wavelength region is expected to open the door to numerous commercial products and processes (requiring 3-12.5 eV photons) that were simply not accessible previously. Furthermore, it is desirable that the spectral breadth of the radiation emitted by such lamps be narrow (less than ˜10 nm) because photochemical processes are renown for their specificity. In other words, a photon of a given wavelength has a specific energy and, therefore, the absorption of a photon by an inorganic or biological molecule yields a product distribution that is also precisely defined. Expanding the spectral bandwidth to, for example, tens of nanometers negates the advantage associated with optically-driven chemical processes and will often result in adverse or competing effects. For example, the phototherapeutic treatment of psoriasis is known to be characterized by a narrow “action spectrum” centered at 308 nm. Irradiating human tissue with photons having wavelengths more than 1-2 nm from this spectral position may be harmful to the patient.
Unfortunately, few commercially-available UV/VUV lamps satisfy both expectations with regard to requirements for average power and spectral bandwidth. A high pressure Hg lamp, for example, is capable of emitting kilowatts of optical power but does so over a broad spectral range (typically 250-580 nm) that does not extend into the VUV region. In contrast, a low-pressure (or “resonance”) Hg lamp emitting at 184.9 nm and 253.7 nm typically generates considerably less than tens of watts of average optical power. Furthermore, the deuterium (D2) molecular lamp emits over a large spectral range and produces little power (<10 W). Another drawback of conventional UV/VUV lamps is their form factor. Generally available in the form of a cylinder, such lamps require expensive reflectors or other optics in order to maximize the efficiency for delivering the UV/VUV radiation to a surface, and for producing a spatially uniform distribution of intensity at that surface.
U.S. Pat. No. 8,900,027 describes a lamp that includes a first and second lamp substrate with a first and second external electrode, respectively, and a first and second internal phosphor coating, respectively. The first phosphor coating is a phosphor monolayer. The method of manufacturing a lamp includes screen-printing a phosphor monolayer on a first lamp substrate; screen-printing a phosphor layer on a second lamp substrate; joining the phosphor coated faces of the first and second lamp substrates together with a seal; and joining a first and second electrode to the uncoupled exterior faces of the first and second lamp substrates, respectively.
U.S. Pat. No. 6,762,556 describes an open chamber photoluminescent lamp. The photoluminescent planar lamp is gas-filled and contains photoluminescent materials that emit visible light when the gas emits ultraviolet energy in response to a plasma discharge. The lamp comprises first and second opposing plates manufactured from a glass material having a loss tangent≤0.05%.
U.S. Publication No. 2002/036461 describes a discharge device for operation in a gas at a prescribed pressure that includes a cathode having a plurality of micro hollows therein, and an anode spaced from the cathode. Each of the micro hollows has dimensions selected to produce a micro hollow discharge at the prescribed pressure. Preferably, each of the micro hollows has a cross-sectional dimension that is on the order of the mean free path of electrons in the gas.